Marshall, 35, provides back-up to Brad Friedel and is in his second spell at the club.
The former Coventry player has still to make his debut under manager Gerard Houllier but he has impressed sufficiently to earn a new deal and has replaced Brad Guzan, now on loan at Hull, as the second-choice goalkeeper.
Marshall, whose current contract ends in June, confirmed: "I'm just about to sign a new contract so it is very pleasing because I really enjoy being at this football club.
"It can be frustrating (being back-up goalkeeper) but I've gone from number three to number two.
"It will be hard to dislodge Brad Friedel who has been phenomenal and it will take an injury, not loss of form. It's not being defeatist, it's being realistic."
Source: PA
Source: PA